Process for polymerizing olefins in a fluidized bed

ABSTRACT

The present invention is directed to a process for polymerizing olefins in gas phase in a fluidized bed reactor having a vertical body, a generally conical downwards tapering bottom zone, a generally cylindrical middle zone above the bottom zone, and a generally conical upwards tapering top zone above the middle zone. The fluidization gas is withdrawn from the top zone of the reactor, compressed and cooled so that a part of the fluidization gas condenses and then introduced to the bottom zone of the reactor. The bed is thus cooled upon evaporation of the liquid. There is no fluidization grid in the reactor.

This application is a national stage application under 35 USC 371 of PCTApplication No. PCT/EP2015/054152 having an international filing date ofFeb. 27, 2015, which is designated in the United States and whichclaimed the benefit of EP Patent Application No. 14157154.7 filed onFeb. 28, 2014, the entire disclosures of each are hereby incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present invention is directed to the polymerization of olefins in afluidized bed reactor. More specifically, the present invention isdirected to the polymerization of olefins in a vertical fluidized bedreactor having no fluidization grid. Even more specifically, the presentinvention is directed to increasing the production capacity inpolymerization of olefins in a vertical fluidized bed reactor having nofluidization grid.

Prior Art and Problem to Be Solved

EP-A-2495037 and EP-A-2495038 disclose a process where olefins arepolymerized in a fluidized bed reactor where the reactor does notcontain a gas distribution plate. The superficial gas velocity withinthe bed in the cylindrical part was reported to be from 0.1 to 0.3 m/s.

While the above-cited prior art discloses a novel process for thepolymerization of olefins they still leave open how to increase theproduction capacity of such processes. The present invention aims toprovide an improved process for polymerizing olefins having an increasedproduction capacity and which can be operated for long periods without ashut-down and where the accumulation of reaction mixture components iseliminated. Further, the process does not produce a substantial amountof agglomerated polymer.

SUMMARY OF THE INVENTION

The present invention provides a process comprising polymerizing atleast one olefin in gas phase in a fluidized bed in the presence of anolefin polymerization catalyst in a polymerization reactor having avertical body, a generally conical downwards tapering bottom zone, agenerally cylindrical middle zone above and connected to said bottomzone, and a generally conical upwards tapering top zone above andconnected to said middle zone and wherein (i) fluidization gas isintroduced to the bottom zone of the reactor from where it passesupwards through the reactor; (ii) the fluidization gas is withdrawn fromthe top zone of the reactor, compressed and cooled so that a part of thefluidization gas condenses thus forming a mixture of liquid and gas, andthe mixture is returned into the bottom zone of the reactor; (iii) afluidized bed is formed within the reactor where the growing polymerparticles are suspended in the upwards rising gas stream; (iv) the bedis cooled upon evaporation of the liquid; and (v) there is nofluidization grid in the reactor; characterized in that the amount ofthe liquid in the mixture introduced into the bottom zone of the reactoris from 0.5 to 20% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified process diagram illustrating the process of thepresent invention.

FIG. 2 is a schematic drawing illustrating the top zone of the reactor.

FIG. 3 is a schematic drawing illustrating the bottom zone of thereactor.

DETAILED DESCRIPTION Definitions

The present text refers to diameter and equivalent diameter. In case ofnon-spherical objects the equivalent diameter denotes the diameter of asphere or a circle which has the same volume or area (in case of acircle) as the non-spherical object. It should be understood that eventhough the present text sometimes refers to diameter, the object inquestion needs not be spherical unless otherwise specifically mentioned.In case of non-spherical objects (particles or cross-sections) theequivalent diameter is then meant.

As it is well understood in the art the superficial gas velocity denotesthe velocity of the gas in an empty construction. Thus, the superficialgas velocity within the middle zone is the volumetric flow rate of thegas (in m³/s) divided by the cross-sectional area of the middle zone (inm²) and the area occupied by the particles is thus neglected.

The olefins polymerized in the process of the present invention aretypically alpha-olefins having from 2 to 10 carbon atoms. Preferably theolefins are ethylene or propylene, optionally together with one or moreother alpha-olefins having from 2 to 8 carbon atoms. Especiallypreferably the process of the present invention is used for polymerizingethylene, optionally with one or more comonomers selected fromalpha-olefins having from 4 to 8 carbon atoms; or propylene, optionallytogether with one or more comonomers selected from ethylene andalpha-olefins having from 4 to 8 carbon atoms.

Unless specifically otherwise defined, the percentage numbers used inthe text refer to percentage by weight.

Catalyst

The polymerisation is conducted in the presence of an olefinpolymerisation catalyst. The catalyst may be any catalyst which iscapable of producing the desired olefin polymer. Suitable catalysts are,among others, Ziegler—Natta catalysts based on a transition metal, suchas titanium, zirconium and/or vanadium catalysts. EspeciallyZiegler—Natta catalysts are useful as they can produce polymers within awide range of molecular weight with a high productivity.

Suitable Ziegler—Natta catalysts preferably contain a magnesiumcompound, an aluminium compound and a titanium compound supported on aparticulate support.

The particulate support can be an inorganic oxide support, such assilica, alumina, titania, silica-alumina and silica-titania. Preferably,the support is silica.

The average particle size of the silica support can be typically from 10to 100 μm. However, it has turned out that special advantages can beobtained if the support has median particle size from 6 to 40 μm,preferably from 6 to 30 μm.

The magnesium compound is a reaction product of a magnesium dialkyl andan alcohol. The alcohol is a linear or branched aliphatic monoalcohol.Preferably, the alcohol has from 6 to 16 carbon atoms. Branched alcoholsare especially preferred, and 2-ethyl-1-hexanol is one example of thepreferred alcohols. The magnesium dialkyl may be any compound ofmagnesium bonding to two alkyl groups, which may be the same ordifferent. Butyl-octyl magnesium is one example of the preferredmagnesium dialkyls.

The aluminium compound is chlorine containing aluminium alkyl.Especially preferred compounds are aluminium alkyl dichlorides andaluminium alkyl sesquichlorides.

The titanium compound is a halogen containing titanium compound,preferably chlorine containing titanium compound. Especially preferredtitanium compound is titanium tetrachloride.

The catalyst can be prepared by sequentially contacting the carrier withthe above mentioned compounds, as described in EP-A-688794 orWO-A-99/51646. Alternatively, it can be prepared by first preparing asolution from the components and then contacting the solution with acarrier, as described in WO-A-01/55230.

Another group of suitable Ziegler—Natta catalysts contain a titaniumcompound together with a magnesium halide compound acting as a support.Thus, the catalyst contains a titanium compound on a magnesium dihalide,like magnesium dichloride. Such catalysts are disclosed, for instance,in WO-A-2005/118655 and EP-A-810235.

Still a further type of Ziegler-Natta catalysts are catalysts preparedby a method, wherein an emulsion is formed, wherein the activecomponents form a dispersed, i.e. a discontinuous phase in the emulsionof at least two liquid phases. The dispersed phase, in the form ofdroplets, is solidified from the emulsion, wherein catalyst in the formof solid particles is formed. The principles of preparation of thesetypes of catalysts are given in WO-A-2003/106510 of Borealis.

The Ziegler—Natta catalyst is used together with an activator. Suitableactivators are metal alkyl compounds and especially aluminium alkylcompounds. These compounds include alkyl aluminium halides, such asethylaluminium dichloride, diethylaluminium chloride, ethylaluminiumsesquichloride, dimethylaluminium chloride and the like. They alsoinclude trialkylaluminium compounds, such as trimethylaluminium,triethylaluminium, tri-isobutylaluminium, trihexylaluminium andtri-n-octylaluminium. Furthermore they include alkylaluminiumoxy-compounds, such as methylaluminiumoxane (MAO),hexaisobutylaluminiumoxane (HIBAO) and tetraisobutylaluminiumoxane(TIBAO). Also other aluminium alkyl compounds, such asisoprenylaluminium, may be used. Especially preferred activators aretrialkylaluminiums, of which triethylaluminium, trimethylaluminium andtri-isobutylaluminium are particularly used. If needed the activator mayalso include an external electron donor. Suitable electron donorcompounds are disclosed in WO 95/32994, U.S. Pat. No. 4,107,414, U.S.Pat. No. 4,186,107, U.S. Pat. No. 4,226,963, U.S. Pat. No. 4,347,160,U.S. Pat. No. 4,382,019, U.S. Pat. No. 4,435,550, U.S. Pat. No.4,465,782, U.S. Pat. No. 4,472,524, U.S. Pat. No. 4,473,660, U.S. Pat.No. 4,522,930, U.S. Pat. No. 4,530,912, U.S. Pat. No. 4,532,313, U.S.Pat. No. 4,560,671 and U.S. Pat. No. 4,657,882. Also electron donorsconsisting of organosilane compounds, containing Si—OCOR, Si—OR, and/orSi—NR₂ bonds, having silicon as the central atom, and R is an alkyl,alkenyl, aryl, arylalkyl or cycloalkyl with 1-20 carbon atoms are knownin the art. Such compounds are described in U.S. Pat. No. 4,472,524,U.S. Pat. No. 4,522,930, U.S. Pat. No. 4,560,671, U.S. Pat. No.4,581,342, U.S. Pat. No. 4,657,882, EP 45976, EP 45977 and EP1538167.

The amount in which the activator is used depends on the specificcatalyst and activator. Typically triethylaluminium is used in suchamount that the molar ratio of aluminium to the transition metal, likeAl/Ti, is from 1 to 1000, preferably from 3 to 100 and in particularfrom about 5 to about 30 mol/mol.

Also metallocene catalysts may be used. Metallocene catalysts comprise atransition metal compound which contains a cyclopentadienyl, indenyl orfluorenyl ligand. Preferably the catalyst contains two cyclopentadienyl,indenyl or fluorenyl ligands, which may be bridged by a group preferablycontaining silicon and/or carbon atom(s). Further, the ligands may havesubstituents, such as alkyl groups, aryl groups, arylalkyl groups,alkylaryl groups, silyl groups, siloxy groups, alkoxy groups or otherheteroatom groups or the like. Suitable metallocene catalysts are knownin the art and are disclosed, among others, in WO-A-95/12622,WO-A-96/32423, WO-A-97/28170, WO-A-98/32776, WO-A-99/61489,WO-A-03/010208, WO-A-03/051934, WO-A-03/051514, WO-A-2004/085499,EP-A-1752462 and EP-A-1739103.

Prior Polymerization Stages

The polymerization in the fluidized bed may be preceded by priorpolymerization stages, such as prepolymerization or anotherpolymerization stage conducted in slurry or gas phase. Suchpolymerization stages, if present, can be conducted according to theprocedures well known in the art. Suitable processes includingpolymerization and other process stages which could precede thepolymerization process of the present invention are disclosed inWO-A-92/12182, WO-A-96/18662, EP-A-1415999, WO-A-98/58976, EP-A-887380,WO-A-98/58977, EP-A-1860125, GB-A-1580635, U.S. Pat. No. 4,582,816, U.S.Pat. No. 3,405,109, U.S. Pat. No. 3,324,093, EP-A-479186 and U.S. Pat.No. 5,391,654. As it is well understood by the person skilled in theart, the catalyst needs to remain active after the prior polymerizationstages.

Polymerization in Fluidized Bed

In the gas phase polymerization reactor the polymerization takes placein a fluidized bed formed by the growing polymer particles in an upwardsmoving gas stream. In the fluidized bed the polymer particles,containing the active catalyst, come into contact with the reactiongases, such as monomer, comonomer(s) and hydrogen which causes polymerto be produced onto the particles.

The polymerization takes place in a reactor including a bottom zone, amiddle zone and a top zone. The bottom zone forms the lower part of thereactor in which the base of the fluidized bed is formed. The base ofthe bed forms in the bottom zone with no fluidization grid, or gasdistribution plate, being present. Above the bottom zone and in directcontact with it is the middle zone. The middle zone and the upper partof the bottom zone contain the fluidized bed. Because there is nofluidization grid there is a free exchange of gas and particles betweenthe different regions within the bottom zone and between the bottom zoneand the middle zone. Finally, above the middle zone and in directcontact therewith is the top zone.

The upwards moving gas stream is established by withdrawing afluidization gas stream from the top zone of the reactor, typically atthe highest location. The gas stream withdrawn from the reactor is thencompressed and cooled and re-introduced to the bottom zone of thereactor. Preferably, the gas is filtered before being passed to thecompressor. Additional monomer, eventual comonomer(s), hydrogen andinert gas are suitably introduced into the circulation gas line. It ispreferred to analyse the composition of the circulation gas, forinstance, by using on-line gas chromatography and adjust the addition ofthe gas components so that their contents are maintained at desiredlevels.

After the compression the gas stream is cooled to remove the heat ofpolymerization and the heat of compression. According to the presentinvention the gas stream is cooled to a temperature where at least apart of it condenses. The components which condense are typically thealpha-olefin monomer or comonomers, such as propylene, 1-butene and1-hexene, or heavier inert components, such as propane, butane, pentaneor hexane. It is possible to add to the fluidization gas a heavier inertcomponent, such as pentane, for increasing the proportion of condensedmaterial in the fluidization gas. The fraction of condensed material inthe fluidization gas is typically from 0.5 to 20% by weight, preferablyfrom 1 to 17% by weight and even more preferably from 1 to 15% byweight. A higher content of condensed material gives improved coolingand thereby increased production rate. On the other hand, an excessamount of condensed material may cause the condensate to accumulate inthe process equipment or in the polymer thereby causing operationalproblems.

The circulation gas line preferably comprises at least one cyclone. Ithas the objective of removing the entrained polymer from the circulationgas. The polymer stream recovered from the cyclone can be directed toanother polymerization stage, or it may be returned into the fluidizedbed reactor or it may be withdrawn as the polymer product.

The bottom zone of the reactor has a generally conical shape taperingdownwards. Because of the shape of the zone, the gas velocity graduallydecreases along the height within said bottom zone. The gas velocity inthe lowest part is greater than the transport velocity and the particleseventually contained in the gas are transported upwards with the gas. Ata certain height within the bottom zone the gas velocity becomes smallerthan the transport velocity and a fluidized bed starts to form. When thegas velocity becomes still smaller the bed becomes denser and thepolymer particles distribute the gas over the whole cross-section of thebed.

Preferably, the equivalent cross-sectional diameter of the bottom zoneis monotonically increasing with respect to the flow direction of thefluidization gas through the fluidized bed reactor. As the flowdirection of the fluidization gas is upwards with respect to the base,the equivalent cross-sectional diameter of the bottom zone is verticallymonotonically increasing.

The bottom zone preferentially has straight circular cone shape. Morepreferably, the cone-angle of the cone-shaped bottom zone is 5° to 30°,even more preferably 7° to 25° and most preferably 9° to 18°, wherebythe cone-angle is the angle between the axis of the cone and the lateralsurface. It is not necessary in this preferred embodiment, however, thatthe bottom zone has the shape of a perfect cone but it may also have ashape of a truncated cone.

The bottom zone may also be seen as being constructed of a plurality ofconical sections having different cone-angles. In such a case it ispreferred that at least the conical section where the base of thefluidized bed is formed has the cone-angle within the above-specifiedlimits. In a most preferred embodiment all the conical sections formingthe bottom zone have the cone-angles within the above-specified limits.If the bottom zone comprises multiple conical sections it is thenpreferred that the steeper sections with a narrower cone angle arelocated at the lower end of the bottom zone and the sections with awider cone angle are located at the higher end of the bottom zone. Sucharrangement is believed to increase the shear forces at the wall of thereactor thus helping to prevent the polymer from adhering to the walls.

It is further preferred that the equivalent diameter of the bottom zoneincreases from about 0.1 to about 1 meters per one meter of height ofthe bottom zone (m/m). More preferably, the diameter increases from 0.15to 0.8 m/m and in particular from 0.2 to 0.6 m/m.

The preferred cone-angles lead to additional improved fluidizationbehaviour and avoid the formation of stagnant zones. As a result, thepolymer quality and stability of the process are improved. Especially, atoo wide cone-angle leads to an uneven fluidization and poordistribution of the gas within the bed. While an extremely narrow anglehas no detrimental effect on the fluidization behaviour it anyway leadsto a higher bottom zone than necessary and is thus not economicallyfeasible.

However, there may be an at least one additional zone being locatedbelow the bottom zone. It is then preferred that the at least oneadditional zone, or if there is more than one additional zone, the totalof the additional zones contributes/contribute to a maximum of 15% tothe total height of the reactor, more preferably 10% to the total heightof the reactor and most preferably less than 5% of the total height ofthe reactor. A typical example of an additional zone is a gas entryzone.

The fluidized bed reactor of the present invention comprises no gasdistribution grid and/or plate. The even distribution of thefluidization gas within the bed is achieved by the shape of the bottomzone. The omission of the gas distribution grid reduces the number oflocations where fouling and chunk formation can start. The terms gasdistribution grid or gas distribution plate or fluidization grid areused synonymously to denote a metal plate or a construction within thereactor which has a purpose of distributing the fluidization gas evenlythroughout the cross-sectional area of the reactor. In the reactorswhere a gas distribution grid is used it generally forms a base of thefluidized bed.

The middle zone of the fluidized bed reactor has a generally cylindricalshape. Preferably it will be in the form of a straight circular cylinderbeing denoted herein simply cylinder. From a more functionalperspective, the middle zone will essentially form a domain wherein thesuperficial velocity of the fluidization gas is essentially constant.

The middle zone typically contains most of the fluidized bed. While thebed extends to the bottom and top zones, its major part is within themiddle zone.

The middle zone has a ratio of the height over diameter (L/D) of atleast about 4, preferably at least about 5. The height over diameter istypically not more than 15, preferably not more than 10.

The gas velocity within the middle zone is such that an effectivecirculation of solids is achieved. This leads to good heat and masstransfer within the bed, which reduce the risk of chunk formation andfouling. Especially, good powder flow near the walls of the reactor hasbeen found to reduce the adhesion of polymer at the wall of the reactor.

It has been found that the gas flow needed to obtain good conditionswithout excess entrainment of polymer from the bed, on one hand, andreduced adhesion of polymer on the walls, on the other hand, depends onthe properties of the polymer powder. Especially for reactors with L/Dof the middle zone of 4 or greater, preferably 5 or greater it has nowbeen found that the gas velocity should preferably be chosen such thatthe dimensionless number, N_(Br), is within the range of from 2.5 to 7,more preferably from 3 to 5. The number N_(Br) can be calculated byusing equation (I):

$\begin{matrix}{N_{Br} = \frac{\frac{d_{90} - d_{10}}{d_{50}}}{\frac{U_{s}}{U_{t}}}} & (I)\end{matrix}$

In equation (I) d₉₀ denotes the smallest equivalent particle diametersuch that 90% of all particles within the bed have a smaller equivalentdiameter than d₉₀; d₁₀ denotes the smallest equivalent particle diametersuch that 10% of all particles within the bed have a smaller equivalentdiameter than d₁₀; d₅₀ represents the median equivalent particlediameter of the particles within the bed; U_(s) is the superficial gasvelocity within the middle zone; and U_(t) is the terminal velocity ofthe particles within the reactor. According to Geldart (Gas FluidizationTechnology, John Wiley & Sons, 1986), equation 6.16, the terminalvelocity in turbulent regime can be calculated from the equation (II)below:

$\begin{matrix}{U_{t} = \sqrt{\frac{4}{3} \cdot \frac{\left( {\rho_{p} - \rho_{g}} \right) \cdot g \cdot d_{v}}{K_{N} \cdot \rho_{g}}}} & ({II})\end{matrix}$

In equation (II) ρ_(p) denotes the particle density (which is the massof the particle divided by its hydrodynamic volume; the volume ofeventual pores is included in the hydrodynamic volume, see explanationsin section 6.12 of Geldart), ρ_(g) is the density of the fluidizationgas, g is the gravity acceleration constant (9.81 m/s²), d_(v) is thevolume diameter of the particles (median volume diameter if theparticles have different diameters), and K_(N) is a correction factor.According to Geldart K_(N) can be calculated from equation (III).K _(N)=5.31−4.88·ψ  (III)

In equation (III) ψ denotes the ratio of the surface area of theequivalent volume sphere to the surface area of the particle, or(d_(v)/d_(s))², where d_(v) is the (median) volume diameter and d_(s) isthe (median) surface diameter of the particle (see Section 2.2 ofGeldart).

The d₉₀, d₁₀ and d₅₀ values are suitably and preferably volume diametersand the percentages 90%, 10% and 50% are based on the mass of theparticles. However, as the ratio is dimensionless it is not absolutelymandatory for d₉₀, d₁₀ and d₅₀ to represent the volume diameter, butthey may also represent another, such as surface per volume or surface,diameter as long as they all represent the same diameter.

It has now been found that the number N_(Br) is a useful characteristicto describe the fluidization regime in the fluidized bed. At low valuesof N_(Br) the bed is in transport conditions. When N_(Br) increases thebed goes over to fluidized conditions, first to entrained fluidization,then bubbling fluidization and finally minimum fluidization.

For low values of N_(Br) of less than 2.5 the bed is in transportconditions. Thereby, depending on the polymer type and particle sizedistribution, a substantial entrainment of polymer from the bed takesplace. Operation in this regime increases the risk of producing finesdue to particle attrition. Powder mixing will be reduced as there ismainly conveying. Cyclone separation efficiency is also reduced and therisk of blocking solids recycling line increases. On the other hand, forhigh values of N_(Br) of greater than 7 the bed is in standard bubblingconditions and then mass and heat transfer within the bed remaininsufficient. The solids mixing may be ineffective, increasing the riskof fouling and agglomeration of particles. The operation of the reactormay become less stable, leading to an increased risk of reactorshut-down.

It has further been found that when N_(Br) is within the above-definedrange then the fraction of the fluidization gas that needs to becondensed is less than what would be needed for higher, moreconventional values of N_(Br) to reach the same cooling capacity. Thisis beneficial because then the eventual problems with accumulation ofcondensed material can be avoided. Furthermore, due to effective powdermixing the conditions within the reactor are homogeneous, i.e., nosignificant temperature gradient is observed.

The height L of the middle zone is the distance of the lowest point ofthe generally cylindrical part of the reactor to the highest point ofthe generally cylindrical part of the reactor. The lowest point of thegenerally cylindrical part is the lowest point above which the diameterof the reactor no longer increases with the height of the reactor butremains constant. The highest point of the generally cylindrical part isthe lowest point above which the diameter of the reactor no longerremains constant with the height of the reactor but decreases. Thediameter D of the middle zone is the (equivalent) diameter of thereactor within the generally cylindrical part.

The top zone of the reactor is shaped such that a gas-particle streamvicinal to the inner walls is created, whereby the gas-particle streamis directed downwards to the base. This gas-particle stream leads to anexcellent particle-gas distribution and to an excellent heat transfer.Further the high velocity of the gas and particles vicinal to the innerwalls minimizes lump- and sheet formation. The top zone has a generallyconical, upwards tapering shape. It is further preferred that the ratioof the height of the top zone to the diameter of the middle zone iswithin the range of from 0.3 to 1.5, more preferably 0.5 to 1.2 and mostpreferably 0.7 to 1.1.

It is particularly preferred that the cone forming the top zone is astraight circular cone and the cylinder forming the middle zonepreferably is a circular cylinder. More preferably the cone-angle of thecone-shaped top zone is 10° to 50°, most preferably 15 to 45°. Asdefined above, the cone-angle is the angle between the axis of the coneand the lateral area.

The specific cone-angles of the cone-shaped upper zone further improvethe tendency for back-flow of the particles countercurrent to thefluidization gas. The resulting unique pressure balance leads to anintensive break up of bubbles, whereby the space-time-yield is furtherimproved. Further as mentioned above, the wall flow velocity, i.e., thevelocity of particles and gas vicinal to the inner walls is high enoughto avoid the formation of lumps and sheets.

Polymer is withdrawn from the reactor. As it was discussed above, onepart of the polymer may be withdrawn by using at least one cycloneinstalled in the circulation gas stream. However, the amount of polymerwithdrawn therefrom is usually not sufficient for the whole polymer tobe withdrawn. Therefore, it is preferred to withdraw polymer also fromthe reactor, especially preferably from the middle zone of the reactor.

The polymer is withdrawn from the middle zone in any manner known in theart, either intermittently or continuously. It is preferred to withdrawthe polymer continuously because then the conditions in the reactorfluctuate less than with intermittent withdrawal. Both methods are wellknown in the art. Continuous withdrawal is disclosed, among others, inWO-A-00/29452, EP-A-2330135 and EP-A-2594433. Intermittent withdrawal isdisclosed, among others, in U.S. Pat. No. 4,621,952, EP-A-188125,EP-A-250169 and EP-A-579426.

In a preferred continuous withdrawal method the polymer is withdrawnthrough an open pipe. In one preferred embodiment the pipe is equippedwith a control valve whose position is automatically adjusted tomaintain a desired outflow rate. The valve position may be set, forinstance, by the reactor bed level controller. In another preferredembodiment the pipe discharges the polymer to a vessel, the pressure ofwhich is controlled to maintain a desired pressure difference betweenthe reactor and the vessel. The pressure difference then sets thepolymer flow rate from the reactor to the vessel.

The agglomerates eventually present in the reactor may be withdrawn byusing one of the powder outlets, as disclosed in EP-A-2594433. However,it is also possible and preferred to withdraw them through a separateoutlet which is preferably located within the bottom zone and suitablybelow the base of the fluidized bed. After recovering the agglomeratesmay be disposed of or they may be crushed and mixed with the product.

Catalyst, which is optionally dispersed within polymer, is introducedinto the reactor, suitably into the fluidized bed. Any method known inthe art may be used for introducing the catalyst. According to onemethod the catalyst, or the polymer containing the catalyst, isintroduced in a stream of inert gas. According to another method thecatalyst is introduced as slurry in a liquid diluent.

It is possible to introduce the catalyst, optionally dispersed withinpolymer particles, also into the bottom zone to the level where the baseof the bed is formed or even below it. The fluidization gas thentransports the particles into the fluidized bed. This is especiallyuseful when the gas phase reactor is preceded by a prior polymerizationstage. The catalyst is then dispersed within the polymer particlesformed in the prior polymerization stage.

Post-Reactor Treatment

When the polymer has been removed from the polymerization reactor it issubjected to process steps for removing residual hydrocarbons from thepolymer. Such processes are well known in the art and can includepressure reduction steps, purging steps, stripping steps, extractionsteps and so on. Also combinations of different steps are possible.

According to one preferred process a part of the hydrocarbons is removedfrom the polymer powder by reducing the pressure. The powder is thencontacted with steam at a temperature of from 90 to 110° C. for a periodof from 10 minutes to 3 hours. Thereafter the powder is purged withinert gas, such as nitrogen, over a period of from 1 to 60 minutes at atemperature of from 20 to 80° C.

According to another preferred process the polymer powder is subjectedto a pressure reduction as described above. Thereafter it is purged withan inert gas, such as nitrogen, over a period of from 20 minutes to 5hours at a temperature of from 50 to 90° C. The inert gas may containfrom 0.0001 to 5%, preferably from 0.001 to 1%, by weight of componentsfor deactivating the catalyst contained in the polymer, such as steam.

The purging steps are preferably conducted continuously in a settledmoving bed. The polymer moves downwards as a plug flow and the purgegas, which is introduced to the bottom of the bed, flows upwards.

Suitable processes for removing hydrocarbons from polymer are disclosedin WO-A-02/088194, EP-A-683176, EP-A-372239, EP-A-47077 andGB-A-1272778.

After the removal of residual hydrocarbons the polymer is preferablymixed with additives as it is well known in the art. Such additivesinclude antioxidants, process stabilizers, neutralizers, lubricatingagents, nucleating agents, pigments and so on.

The polymer particles are mixed with additives and extruded to pelletsas it is known in the art. Preferably a counter-rotating twin screwextruder is used for the extrusion step. Such extruders aremanufactured, for instance, by Kobe and Japan Steel Works. A suitableexample of such extruders is disclosed in EP-A-1600276.

Benefits of the Invention

The present invention can be operated for long periods of time withreduced down-time. The polymerization takes place in homogeneousconditions, i.e., in the absence of regions having different gascomposition or temperature than the average of the bed. The adhesion ofpolymer on the wall and other parts of the reactor are avoided. Theprocess can be operated with a high production capacity withoutaccumulation of reactants or by-products in the process equipment.Thereby good product consistency and economical operation are obtained.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reactor system according to the present invention. Thereactor (1) has a bottom zone (5), a middle zone (6) and a top zone (7).The fluidization gas is introduced into the bottom zone (5) through theopening (8). While the gas flows upwards through the bottom zone (5) itssuperficial velocity reduces due to the increasing diameter. A fluidizedbed starts to form within the bottom zone (5). The gas continues totravel upwards through the middle zone (6) where the gas velocity isconstant and the bed is fully formed. Finally the gas reaches the topzone (7) from where it is withdrawn through the opening (9). The gas,together with entrained solids, passes along line (12) to a cyclone (2).The cyclone (2) removes most of the entrained solid from the circulationgas which is passed through the gas outlet (13) along the lines (16) and(18) to a compressor (17). Before the compressor (17) there ispreferably a filter (4). In the compressor (17) the gas is pressurizedand passed through line (19) to a cooler (3) where it is cooled. Fromthe cooler (3) the gas is passed along the line (20) into the inlet (8)of the reactor (1).

The solid stream is passed from the cyclone (2) through the opening (14)to line (21). By using a valve (15) the solid stream may be eitherwithdrawn and sent to further processing along line (23) or returnedinto the reactor (1) along line (22) through the opening (24).

The polymer is product is withdrawn from the reactor (1) along one ormore outlets (11). Catalyst, optionally dispersed within polymerparticles from a preceding polymerization stage, is introduced into thereactor (1) along line (10). Additional monomer, comonomer, hydrogen andinert gas may be introduced at a convenient location of the circulationgas line (16, 18, 19, 20).

FIG. 2 is a schematic drawing of the top zone (7). Within the top zone,usually at its highest location, there is an opening (9) for withdrawingcirculation gas from the reactor.

FIG. 3 shows analogously the bottom zone.

EXAMPLES

In Examples 1 to 3 the reactor was operated at an absolute pressure of20 bars and a temperature of 85° C. Propane was used as the fluidizationgas and the bed is formed of polyethylene particles having an averagediameter of 250 μm with polymer production rate of 90 kg/hr. Polymerparticles from a prior polymerization stage were continuously introducedinto the gas phase reactor and polymer was continuously withdrawn fromthe middle zone to maintain a constant bed level. The resultingpolyethylene had a density of 923 kg/m³ and MFR₅ of 0.24 g/10 min. Thereactor had the following properties:

Height of the bottom zone: 900 mm

Height of the middle zone: 2700 mm

Height of the upper zone 415 mm

Diameter of the middle zone 540 mm

Example 1

The reactor as described above was operated so that flow rate of thefluidization gas was 250 m³/h. The bed was filled with polyethylene witha filling degree of about 60% of the volume of the middle zone. Thesuperficial gas velocity at the gas inlet, where the diameter of thereactor was 100 mm, was 8.5 m/s and in the middle zone 0.3 m/s. It couldbe seen that small size bubbles were formed; these bubbles travelledalong the whole reactor length. In order to control reactor temperatureat 85° C., the condensate content of the fluidization gas at reactorinlet was kept around 12% by weight.

Example 2

The reactor as described above was operated so that flow rate of thefluidization gas was 412 m³/h. The bed was filled with polyethylene witha filling degree of about 60% of the volume of the middle zone. Thesuperficial gas velocity at the gas inlet, where the diameter of thereactor was 100 mm, was 14.5 m/s and in the middle zone 0.5 m/s. Itcould be seen that medium size bubbles were formed; these bubblestravelled along the whole reactor length. In order to control reactortemperature at 85° C., the condensate content of the fluidization gas atreactor inlet was kept around 7% by weight.

Example 3

The reactor as described above was operated so that flow rate of thefluidization gas was 580 m³/h. The bed was filled with polyethylene witha filling degree of about 60% of the volume of the middle zone. Thesuperficial gas velocity at the gas inlet, where the diameter of thereactor was 100 mm, was 20.5 m/s and in the middle zone 0.7 m/s. Itcould be seen that large size bubbles were formed; these bubbles havecomparable size to reactor diameter and it travelled along the wholereactor length. In order to control reactor temperature at 85° C., thecondensate content of the fluidization gas at reactor inlet was keptaround 1.5% by weight.

TABLE 1 Conditions in Examples 1 to 3 1 2 3 Flow rate of fluidizationgas, tn/h 10.5 17.3 24.4 Bed height*, mm 1650 1650 1650 Filling degreeof reactor**, % 60 60 60 Superficial gas velocity, m/s 0.3 0.5 0.7 NBr7.9 4.8 3.4 Reactor temperature, ° C. 85 85 85 Bubbles' size smallmedium large Degree of condensation, wt % 12 7 1.5 *Starting from planeseparating bottom and middle zones **With respect to the volume of themiddle zone

The invention claimed is:
 1. An olefin polymerization process comprisingpolymerizing at least one olefin in gas phase in a fluidized bed in thepresence of an olefin polymerization catalyst in a polymerizationreactor having a vertical body, a generally conical downwards taperingbottom zone, a generally cylindrical middle zone above and connected tosaid bottom zone, and a generally conical upwards tapering top zoneabove and connected to said middle zone and wherein (i) fluidization gasis introduced to the bottom zone of the reactor from where it passesupwards through the reactor; (ii) the fluidization gas is withdrawn fromthe top zone of the reactor, compressed and cooled so that a part of thefluidization gas condenses thus forming a mixture of liquid and gas, andthe mixture is returned into the bottom zone of the reactor; (iii) afluidized bed is formed within the reactor where the growing polymerparticles are suspended in the upwards rising gas stream; (iv) the bedis cooled upon evaporation of the liquid; and (v) there is nofluidization grid in the reactor; characterized in that the amount ofthe liquid in the mixture introduced into the bottom zone of the reactoris from 0.5 to 20% by weight.
 2. The process according to claim 1characterized in that the gas velocity is maintained in the reactor suchthat N_(Br) is within the range of from 2.5 to 7 wherein$N_{Br} = \frac{\frac{d_{90} - d_{10}}{d_{50}}}{\frac{U_{s}}{U_{t}}}$wherein do represents the smallest equivalent particle diameter so that90% of the particles have a smaller equivalent diameter than d₉₀; d₁₀represents the smallest equivalent particle diameter so that 10% of theparticles have a smaller equivalent diameter than d₁₀; d₅₀ representsthe median equivalent particle diameter; U_(s) is the superficial gasvelocity within the middle zone; and U_(t) is the terminal velocity ofthe particles within the reactor.
 3. The process according to claim 2characterized in that N_(Br) is within the range of from 3 to
 5. 4. Theprocess according to claim 1 wherein the amount of the liquid in themixture introduced into the bottom zone of the reactor is from 1 to 17%by weight.
 5. The process according to claim 1 wherein the ratio L/D ofthe height of the middle zone, L, to the diameter of the middle zone, D,is from 4 to
 15. 6. The process according to claim 5 wherein the ratioL/D is from 5 to
 10. 7. The process according to claim 1 comprising thestep of removing polymer from the fluidization gas which has beenwithdrawn from the top zone of the reactor before the compression andcooling steps.
 8. The process according to claim 7 wherein the polymerremoved from the fluidization gas is recovered and passed to furtherprocessing.
 9. The process according to claim 7 wherein the polymerremoved from the fluidization gas is returned to the polymerizationreactor.
 10. The process according to claim 1 wherein the fluidizationgas is filtered before being passed to the compressor.
 11. The processaccording to claim 1 wherein polymer is withdrawn from the reactorthrough an outlet located in the middle zone.
 12. The process accordingto claim 11 wherein the polymer is withdrawn from the reactorcontinuously.
 13. The process according to claim 11 wherein polymer orpolymer agglomerates are withdrawn from the bottom zone of the reactor.14. The process according to claim 1 wherein the bottom zone comprisesmultiple conical segments having different cone angles.
 15. The processaccording to claim 12 wherein polymer or polymer agglomerates arewithdrawn from the bottom zone of the reactor.